This invention relates to vulcanizable semi-conductive compositions which provide strippable semi-conductive insulation shielding compositions for insulated electrical conductors.
The construction of insulated electrical conductors, i.e., wires and cable, designed for medium to high voltage applications is well known in the art and commonly comprises a core conductor which comprises one or more strands or a conducting metal or alloy such as copper or aluminum, a layer of semi-conductive shielding, a layer of insulation, such as cross-linked polyethylene, and a layer of semi-conductive insulation shielding overlying said insulation. A plurality of neutral wires which are usually made of copper or aluminum may be embedded in or wrapped around the layer of semi-conducting insulation shielding, if desired, in the form of a concentric ring around the insulated cable.
The insulation layer and its overlying semi-conductive shielding layer can be formed in the art by what is known as a two pass operation or by a single pass triple extrusion process. The two pass operation is one in which the insulation is first extruded and crosslinked prior to extrusion and crosslinking of the semi-conductive insulation shielding layer. In the single pass triple extrusion operation (sometimes called a tandem extrusion when referring only to the insulation layer and its semi-conductive shielding layer) the semi-conductive conductor shielding layer, the insulation layer, and the overlying semi-conductive insulation shielding layer are extruded in sequence and cured (crosslinked) simultaneously in a single operation to minimize manufacturing steps. For obvious reasons the single pass triple extrusion method is preferred by manufacturers. However, the sumultaneous curing of the insulation layer and its overlying semi-conductive shielding layer of the triple extrusion method in general makes the shielding layer more fully bonded in most cases to the insulation that it might be if it were made as a result of a two pass operation.
The formation of the bond between the insulation and insulation shielding layer makes subsequent separation of the two layers (insulation and semi-conductive shielding) such as occurs in making splices or terminal connections, very difficult and time consuming. Such a strong bond also makes the semi-conductive layer prone to leave carbon residue on the insulation even when it is finally peeled off. Accordingly, a strippable semi-conductive shielding which can be easily and cleanly stripped from the insulation of an insulated conductor that has been made by a single pass triple extrusion operation is therefore very desirable in this art.
It has now been discovered that such types of easily strippable semi-conducting insulation shielding compositions can be prepared from the vulcanizable semi-conductive insulation shielding compositions of this invention which contain both an ethylene-vinyl acetate copolymer and polyethylene homopolymer as described more fully below, the cured insulation shieldings of which have been found to be more easily strippable from the cured insulation than that of corresponding cured insulation shieldings obtained from vulcanizable ethylene-vinyl acetate copolymer compositions free of polyethylene homopolymer even when both the insulation and insulation shielding layers are cured simultaneously in one operation.
Thus, it is an object of this invention to provide a vulcanizable semi-conductive insulation shielding composition which is particularly useful for providing a strippable shielding for insulated electrical conductors, e.g., wires and cables, that contain, as the primary insulation, a crosslinked polyolefin. Another object of this invention is to provide an article of manufacture comprising the crosslinked product of said vulcanizable semi-conductive insulation shielding composition of this invention bonded directly to a crosslinked polyolefin substrate. Yet another object of this invention is to provide an article of manufacture as defined above wherein said crosslinked polyethylene is the primary insulation of an insulated electrical conductor, thus providing insulated electrical conductors, e.g., wires and cables, comprising, as the primary insulation, a crosslinked polyolefin, and as the shielding for said insulation an easily strippable crosslinked semi-conductive shielding composition. Other objects and advantages of this invention will become readily apparent from the following description and appended claims.
More specifically, the instant invention may be described as a strippable vulcanizable semi-conductive insulation shielding composition consisting essentially of, based on the total weight of said composition, (A) about 40 to about 90 weight percent of an ethylene-vinyl acetate copolymer containing from about 27 to 45 weight percent of vinyl acetate based on the total weight of said copolymer, (B) about 3 to about 15 weight percent of a low density, low molecular weight polyethylene homopolymer, (C) about 8 to about 45 weight percent of carbon black, and (D) about 0.2 to about 5 weight percent of an organic peroxide crosslinking agent.